[InstCombine] visitPHINode(): use InstCombiner::replaceInstUsesWith() instead of RAUW

As noted in post-commit review, we really shouldn't use RAUW in InstCombine
because we should consistently update Worklist to avoid extra iterations.

llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp

@@ -1439,7 +1439,7 @@ Instruction *InstCombinerImpl::visitPHINode(PHINode &PN) {
         continue;
       // Just use that PHI instead then.
       ++NumPHICSEs;
-      PN.replaceAllUsesWith(&Src);
+      replaceInstUsesWith(PN, &Src);
       return &PN;
     }
   }